The present invention relates to a device for controlling the quantity of hydraulic fluid, e.g. oil, returned to a pump from a double-acting lifting cylinder employed for lifting and lowering a load (e.g. a truck-tilt cab) which moves through a dead point.
When tilting a cab, hydraulic pressure is exerted against a piston of a lifting cylinder by a pump, which is usually a manually operated pump. The hydraulic pressure is required until the centre of gravity of the cab has been lifted so far, that it is in a vertical position above the pivotal centre of the cab. If the cab is tilted further, hydraulic fluid under pressure is still supplied by the pump, but the cabin starts to release its potential energy. This energy should be absorbed by the flow resistance of the hydraulic fluid.
In tilt cabs of the above described type, it is required that they be able to remain in any position, i.e. that the tilting movement may only take place if the pump is actually operated. Moreover, it is then required that in any position, in which the cab is located, leakage of hydraulic fluid is excluded. This means that the valve or valves in the hydraulic control mechanism have to lock completely.
Another demand which is made on a control device for such a cylinder is that when the pump is operated during that part of the stroke in which the cab is being lifted, the flow of hydraulic fluid should take place with as little resistance as possible in order to keep the force required for tilting as small as possible.
In known constructions, in order to satisfy the demand that the locking of the flow of hydraulic fluid at a certain moment takes place in a completely leakage-free manner, a completely locking valve (for instance a ball valve) is used as a locking member. Such a valve construction, however, is usually not controllable, i.e. the flow is either completely blocked or completely or almost completely opened.
When the energy of the pump and of the cab, after passing the dead point during tilting, have to be taken up by flow resistance in the hydraulic control system, a valve which has only an opened or closed position has serious disadvantages. The cab will first make a free fall by which the valve abruptly closes while the movement of the cab is thus suddenly braked, thereby causing a violent jolting. This will soon lead to rupture of the suspension points.